S45C vs 4140 Steel: Which Steel for Your Part

OpusFab lists two steels for CNC machining: S45C and 4140 Pre-Hardened. Both are strong, both machine well, and neither resists corrosion on its own. The choice between them comes down to how much wear and load the part will see, and what you are willing to pay.

S45C: the general engineering steel

S45C is a medium-carbon steel (around 0.45 percent carbon) used across general engineering. It is strong, stiff, and one of the most economical metals in the catalogue, both for stock and for machining time.

Characteristics:

• Good strength and rigidity for structural and mechanical parts
• Machines cleanly and quickly, so part cost stays low
• No corrosion resistance: bare S45C will rust, so plan a finish for anything that sees moisture
• Can be zinc or chrome plated directly in the editor

Typical uses: shafts, gears, jigs, fixtures, machine frames, and structural brackets.

4140 Pre-Hardened: the wear and load steel

4140 is a chromium-molybdenum alloy steel, supplied pre-hardened to around 28-32 HRC. That hardness arrives with the stock, so parts come off the machine ready for service with no heat treatment step.

Compared to S45C, 4140 is noticeably tougher and more wear resistant. It holds up in high-stress and sliding-wear applications where S45C would deform or wear through. The trade-offs are price and cycle time: the stock costs more, and harder material machines slower, which raises the part cost further.

Typical uses: high-load shafts, gears, axles, tooling, dies, and wear plates. Like S45C, it can be zinc or chrome plated in the editor.

Quick comparison

	S45C	4140 Pre-Hardened
Type	Medium-carbon steel	Chromium-molybdenum alloy steel
Hardness	As-rolled, moderate	Pre-hardened, ~28-32 HRC
Wear resistance	Moderate	High
Machining	Fast and easy	Slower, more tool wear
Cost	Low	Higher
Finishes	Zinc plate, chrome plate, sandblast, PVD, powder coat	Zinc plate, chrome plate, sandblast, PVD, powder coat

Neither is stainless

Both steels rust in moisture or outdoor use. Zinc plating gives economical corrosion protection with a bright silvery finish; chrome plating adds a harder, brighter surface. For anything beyond mild exposure, or where you do not want to rely on a coating at all, a stainless grade is the better pick: see Stainless Steel Options.

When to choose S45C

• General mechanical and structural parts where cost matters
• Jigs, fixtures, and tooling used indoors
• Parts that will be zinc or chrome plated anyway
• Anything where the loads are moderate and wear is not the failure mode

When to choose 4140

• Shafts, gears, and axles under real load
• Wear surfaces and sliding contact
• Tooling and dies that need to keep an edge
• Parts where a failure would be expensive or unsafe

If neither fits, aluminium is lighter and corrosion-resistant (see Aluminium 6061 vs 7075), and the full metal list is in Materials overview.